fping 5.6: some code refactoring

[schweikert]

After fping 5.5 is released and after the traceroute code is merged, we could do some refactorings. I have in mind:

1. Consistent variable names for command-line options. Proposal: "opt_XXXX" (e.g. opt_count) for values and "opt_XXXX_on" (e.g. opt_count_on) for on/off switches.
2. Move option parsing to a separate file (e.g. flags.c and flags.h).
3. Move printing functions to a separate file (e.g. output.c / output.h)
4. Move stats functions (global stats) to a separate file (e.g. stats.c / stats.h)

[gsnw-sebast]

I'm starting with point 1